A gas turbine engine includes an elongated member designed to rotate about its longitudinal axis. This rotating member is located within a generally tubular casing which remains stationary relative to the rotating member, with the longitudinal axes of the rotating member and the tubular casing being colinear. The rotating member includes a plurality of blade structures fixedly attached to the rotating member, and disposed along the longitudinal axis of the rotating member. In turn, each blade structure includes a plurality of blades disposed radially about the rotating member. Disposed between adjacent blade structures and attached to the stationary tubular casing are annular nozzle structures. Each annular nozzle structure includes a plurality of vanes disposed about the nozzle structure. Hot pressurized gas directed into the turbine engine is further directed by the vanes on the nozzle structures to strike the blades on the rotating member at an optimum angle causing the rotation of the rotating member.
The effectiveness of the turbine engine varies directly with the proportion of gas that impinges upon the blades of the rotating member. However, a certain amount of gas unavoidably leaks through the space between the outermost points of the rotating members, or rotors, and the innermost points of the stationary members, or stators. This flow of gas through the space between the rotors and stators is caused partly by the general movement of gas through the turbine engine, and partly by the fact that gas pressure on the downstream side of a rotor or stator is lower than the gas pressure on the upstream side.
The amount of gas that escapes between the rotor and stator is reduced by decreasing the distance between the rotor and stator. However, it is well known that for any given clearance, the amount of gas flowing between the rotor and stator can be reduced even further by providing a structure that will create turbulence in the gas flowing between the rotor and stator. For example, a plurality of spaced annular knife edges on either the rotor or stator defines a labyrinth seal which is known to create a turbulence that will inhibit the flow of gas between the rotor and stator. It is also known that as the space between successive annular knife edge structures in the labyrinth seal increases, the amount of turbulence created thereby also increases.
It is known that a honeycomb structure will cause a turbulent effect similar to that described above for the labyrinth seal. More specifically, the honeycomb structure is attached to either the rotor or stator so that the longitudinal axis of each honeycomb cell is aligned in a generally radial direction. As with labyrinths, the turbulence created increases with the width of each honeycomb cell. The effectiveness of the honeycomb seal also varies with the depth of the cells, and the radial distance, or gap, between the rotor and stator that is not filled by the honeycomb structure. Typically this gap would be in the range of 0.005 inches to 0.040 inches. The size of the gap selected would depend on the width of the individual honeycomb cells.
The attributes of various seals are explained in detail in the publication of C.A. Mayer and J. A. Laurie III "The Leakage Thru Straight and Slant Labyrinth and Honeycomb Seals" Journal of Engineering for Power (Oct. 1975), the disclosure of which is incorporated herein by reference. For example, the aforementioned publication presents data describing the discharge coefficient of a turbine engine for various seal configurations and at various radial seal clearances between the stationary and moving parts of the turbine engine. Specifically for a radial seal clearance of 0.020 inches and for a 0.5 ratio of static pressure downstream to total pressure upstream various discharge coefficients can be compared under these conditions a honeycomb seal with 0.187 inch wide 0.750 inch long cells had a discharge coefficient of 0.50; a honeycomb seal with 0.125 inch wide 0.750 inch long cells had discharge coefficient of 0.59; a three-labyrinth straight seal 1.00 inch long with a 0.50 inch pitch had a discharge coefficient of 0.62; and a single labyrinth seal had a discharge coefficient of 0.077. Thus the discharge coefficient for wide honeycomb cells was only 85 percent of the value for narrower cells, and only 64 percent of the value for the single labyrinth seal. Hence it is apparent that the effectiveness of a turbine engine varies inversely with the discharge coefficient. As a result, it is apparent that honeycomb seals with wide cells offer substantial operational benefits.
Many honeycomb arrangements and methods for making honeycomb structures are known, including those disclosed in: U.S. Pat. No. 2,963,307 which issued to Bobo on Dec. 6, 1960; U.S. Pat. No. 3,046,648 which issued to Kelly on July 31, 1962; U.S. Pat. No. 3,603,599 which issued to Laird on Sept. 7, 1971; U.S. Pat. No. 4,063,742 which issued to Watkins on Dec. 20, 1977; U.S. Pat. No. 4,162,077 which issued to Crow et al on July 24, 1979; and U.S. Pat. No. 4,218,066 which issued to Ackerman on Aug. 19, 1980. Generally, the prior art patents cited above provide a planar honeycomb sheet, with the longitudinal axis of each honeycomb cell being generally perpendicular to the plane of the sheet. This planar array of honeycomb cells is subsequently bent into a tubular configuration, and is affixed to either the rotor or stator of the turbine engine. However, since the circumference varies directly with the diameter of a circle, the width of an individual cell measured at the radially outer surface of the tubular structure will be greater than the width of the same cell measured at the radially inner surface. Thus the outermost part of the tubular structure is subjected to a tension force, while the innermost part is subjected to compression.
Honeycomb seals with the structural features described above are acceptable for use in a large turbine engine because the difference between the inner and outer circumferences on seals for large turbine engines is small compared to the total circumference. However, in recent years small turbine engines have become increasingly desirable because of their size and weight characteristics. Specifically, in many applications the diameter onto which the seal is mounted may be less than five inches.
It has been observed that planar sheets of honeycomb structures are not readily adaptable to small diameter turbine engines, because the difference between the inner and outer circumferences on seals for small turbine engines is great compared to the total circumference. Therefore, the tension and compression forces to which individual honeycomb cells are subjected also is great. As a result, when a conventional planar honeycomb structure is bent into a tubular configuration for use in a small diameter turbine engine, the honeycomb structure tends to warp, whereby the diameter of the resultant tubular honeycomb structure varies along its length. This phenomenon reflects the tendency of the structure to accommodate the differential tensile and compressive forces by expanding and contracting about more than one axis. If, in forming the tubular structure, the circumferential expansion and contraction is confined to variations about the longitudinal axis of the tube, individual cells and joints between adjacent cells will be more susceptible to failure in small diameter applications. For example, the part of a honeycomb cell on the inside of the tubular structure may tend to buckle under the compression force. Conversely, the part of each cell on the outside will tend to elongate under tension, causing possible failure of the cell wall or the joints between cells.
One approach to overcoming these problems in a small diameter turbine engine is to provide a honeycomb structure with short and narrow individual honeycomb cells. For example, a planar honeycomb sheet may be bent into a tubular configuration having an outside diameter of five inches, if individual cells have a length of less than 0.250 inches and a cross sectional width of less than 0.062 inches. By providing short cells, such as this, the difference between the inner and outer diameters of the tubular structure will be small; and therefore, the difference between the inner and outer circumferences also will be small. As a result, the differential expansion and contraction may be accommodated by the honeycomb structure. Similarly, if individual cell widths are small, there will be more cells disposed about the circumference, and the amount of circumferential variation imposed upon each cell can readily be accommodated. However, as mentioned above, honeycomb cells with large cross sectional widths, typically in the range of 0.125 inches to 0.375 inches are more effective in reducing gas leakage then cells with small cross sections. Additionally, it is often desirable to provide deep cells which are partially filled with an insulating material to reduce the heating of areas adjacent to the turbine engines. However, the differential tension and compression on the inner and outer surfaces of the prior art honeycomb structures had prohibited wide and long cells for honeycomb seals on small turbine engines. For these reasons, the small diameter turbine engines have been used almost exclusively with the labyrinth selas described above.
Accordingly, it is an object of the present invention to provide a honeycomb seal that is adaptable to either large or small turbine engines.
It is a further object of the subject invention to provide a honeycomb seal structure that can be adapted to any individual cell size.
It is still a further object of the subject invention to provide a honeycomb seal structure that retains its dimensional stability in applications on small turbine engines.
It is another object of the subject invention to provide a honeycomb seal structure that can be readily manufactured and assembled.
Another object of the subject invention is to provide a honeycomb seal structure with enhanced structural integrity.